1. Field of the Invention
The present invention relates to frequency-shift type direct relay equipment employed for microwave multiplex radio relay systems.
2. Description of the Prior Art
The development of field effect transistors for microwave amplification made it possible to directly amplify the microwave signals, that is, making it possible to form a direct relay device for directly amplifying and relaying microwave signals in microwave multiplex radio systems. However, in this type of direct relay equipment, the receiving frequency is different from the transmitting frequency. Therefore, the received signals must be amplified through a microwave amplifier, converted into a transmitting frequency through a mixer, and the converted signals amplified and transmitted. Thus, in this type of equipment, an up circuit and a down circuit each need two amplifiers, that is, the direct relay circuit as a whole needs four amplifiers, to amplify the received signals and the transmitted signals, respectively. Microwave amplifiers are expensive components, therefore the direct relay equipment becomes expensive no matter how its construction is simplified.
It was proposed to amplify the received signals and the transmitted signals having a shifted frequency through a single microwave amplifier. However, problems arose with regard to interference between the received signals and the transmitted signals, and strict characteristics are required for the band-pass filters. These problems have not been solved as yet.